Oxide resistor

ABSTRACT

A composite sintered oxide resistor comprising crystal grains of zinc oxide and crystal grains of a zinc oxide compound of other metal or semi-metal element than zinc, and a grain boundary layer having an electric resistance equal to or lower than that of the crystal grains and which of zinc oxide between the individual crystal grains has a very large withstanding capacity against switch surge, a small non-linear coefficient of voltage in the voltage-current characteristics, a positive, smaller resistance-temperature coefficient, and a small percent change in resistivity after heat treatment at 500° C. in the atmosphere.

BACKGROUND OF THE INVENTION

This invention relates to an oxide resistor, and particularly to anoxide resistor suitable for absorption of switching surge of a circuitbreaker, etc.

As to so far known linear resistors for the circuit breaker, there havebeen proposed aluminum oxide-clay-carbon-based compositions having suchcharacteristics as a withstanding capacity against the breaker switchingsurge of 200 Joules/cc, which will be hereinafter referred to as "J/cc",a resistance-temperature coefficient of -9×10⁻² Ω/°C. (20°-250° C.) andan application temperature of 200° C. with a resistivity of about 400Ω-cm.

With recent higher transmission voltage, a linear resistor of smallersize and lighter weight has been desired for the circuit breaker, andthus it has been required that (1) the resistor has a largerwithstanding capacity aginst the switching surge, (2) the resistor has aless fluctuation in resistivity, even if exposed to a high temperature,since the temperature is elevated by exposure to breaker switchingsurges, and (3) the resistor must be made from materials having asmaller resistance-temperature coefficient. The conventional resistor ismade from an aluminum oxide-clay-based material by adding carbonthereto, and by sintering the mixture in an inert gas atmosphere tocontrol the resistivity through the carbon content, and thus has suchdisadvantages that (1) the density of sintered product is low and thewithstanding capacity against the switching surge is small, (2) thecarbon having control of the resistivity is oxidized when the resistoris exposed to a high temperature, resulting in a large fluctuation inthe resistivity, and (3) the resistance-temperature coefficient islarge.

It is known to use a zinc oxide-based resistor in the circuit breaker[Japanese Patent Application Kobai (Laid-open) No. 55-57219], where thesaid requirements (1) to (3), particularly the increase in thewithstanding capacity against the switching surge, have not beeninvestigated.

As a result of extensive studies of crystal grains in sintered productsthat form resistors, the present inventors have successfully satisfiedthe said requirements.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an oxide resistorhaving such characteristics as a resistivity of 40 to 1,000 Ω-cm, alarge withstanding capacity against the breaker switching surge, nofluctuation in the resistivity even if exposed to a temperature of 500°C. or higher, and a low resistance-temperature coefficient.

Another object of the present invention is to provide an oxide resistorhavig a resistance-temperature coefficient ranging from -1×10⁻³ Ω/°C. to+4×10⁻³ Ω/°C.

The present oxide resistor is a composite oxide sintered productcomprising crystal grains of zinc oxide and crystal grains of zinc oxidecompound of other metal or semi-metal element than zinc, and having nograin boundary layer of higher electric resistance than that of thecrystal grains of zinc oxide between the individual crystal grains.Furthermore, the present oxide resistor is a composite sintered productcomprising crystal grains of zinc oxide and crystal grains having anelectric resistance of 200 Ω to 3×10¹³ Ω, and having no grain boundarylayer of higher electric resistance than that of the crystal grains ofzinc oxide, the sintered product being in a plate form including a discform and having electrodes at both end surfaces.

Among the individual crystal grains, there may be a grain boundary layerhaving an electric resistance equal to that of the crystal grains ofzinc oxide, and there may be voids at positions corresponding to thoseof the grain boundary layers among the crystal grains. The voids includea complete absence of the grain boundary layers.

It is desirable that the crystal grains of zinc oxide compound have aresistance of 200 Ω to 3×10¹³ Ω, which is higher than that of zincoxide. It is also desirable that the zinc oxide compound is selectedfrom compounds having the following chemical formulae: Zn₂ TiO₂, Zn₂SiO₄, Zn₂ Sb₂ O₁₂, Zn₂ ZrO₄, and Zn₂ SnO₄. The said metal and semi-metalfor forming these compounds are titanium (Ti), silicon (Si), antimony(Sb), zirconium (Zr), and tin (Sn). It is not desirable to use bismuth(Bi), because a grain boundary layer having a higher resistance isliable to be formed from Bi.

The raw materials for the sintered product are zinc oxide (ZnO) as themajor component and other metal or semi-metal oxides than ZnO as theminor components, such as titanium oxide (TiO₂), silicon oxide (SiO₂),antimony oxide (Sb₂ O₃), zirconium oxide (ZrO₂) and tin oxide (SnO₂).

The structure of the present sintered product is characterized by mutualrelationship between the crystal grains, and can be prepared by properlyselecting the amounts of the components, pressure, temperature, time andincreasing or decreasing rate of temperature in view of the rawmaterials to be used. The resulting resistors generally show alinearity, but in the case of non-linearity it is effective to break thehigh resistance parts, particularly grain boundary layer, by applying ahigh voltage thereto.

As a result of extensive studies of making the breaker resistors smallerin the size and lighter in weight, the present inventors have found that(1) the applicable resistor must have a resistivity of 40 to 4,000 Ω-cm,a withstanding capacity against the switching surge of 400 J/cc or more,a resistance-temperature coefficient in a range of ±1×10⁻³ Ω/°C. (20° to500° C.), and a fluctuation in resistivity of being within ±10% evenafter exposed to a temperature of 500° C. or higher, and (2) thewithstanding capacity against the switching surge of the resistordepends on formation of many kinds of crystal grains having variousresistivities in the resistor and the density of the resistor. Thus, theraw materials for the resistor may be readily sinterable and must formnew crystal grains having different electric resistance through reactionof the raw materials themselves, and the resulting sintered product musthave a high density. Thus, the present inventors have investigatedcharacteristics of resistors comprising zinc oxide, titanium oxide, andmagnesium oxide as the basic components, and further containing antimonyoxide, silicon oxide, zirconium oxide, tin oxide, etc., and consequentlyhave found that (1) the withstanding capacity against the switchingsurge is 800 J/cc which is considerably high, that is, about 4 timesthat of the conventional product, (2) the resistance temperaturecoefficient can be improved through a change from negative to positiveby the content of magnesium oxide (MgO) in the basic components, zincoxide (ZnO), titanium oxide (TiO₂), and magnesium oxide (MgO), and (3)the resistivity can be improved by adding antimony oxide (Sb₂ O₃),silicon oxide (SiO₂), zirconium oxide (ZrO₂), tin oxide (SnO.sub. 2),etc. to the basic components, ZnO, TiO₂ and MgO.

Preferable basic composition for the present resistor comprises 65 to94.8% by mole of ZnO, 5 to 20% by mole of TiO₂, and 0.2 to 15% by moleof MgO. Furthermore, 0.2 to 15% by weight of at least one of such oxidesas Sb₂ O₃ (0.05 to 5% by mole), SiO₂ (0.2 to 23% by mole) and ZrO₂ (0.1to 11% by mole) may be added to the basic composition. When the contentof TiO₂ is above or below the said composition range, theresistance-temperature coefficient goes beyond the range of ±1×10⁻³Ω/°C., and such a resistor is not suitable for the circuit breaker.However, the withstanding capacity against the switching surge can beconsiderably improved by the presence of TiO₂, because it seems that acrystal Zn₂ TiO₄ can be formed by sintering of ZnO and TiO₂ in the rawmaterials, and this crystal has an electric resistance of about 200 to500 Ω, which is a little higher than 10-50 Ω of the ZnO crystal, andcontributes to an improvement of the density of sintered product. MgOcan change the resistance-temperature coefficient from negative topositive, and at least the resistance-temperature coefficient goesbeyond the range of ±1×10⁻³ Ω/°C., when the content of MgO is above orbelow the said composition range as in the case of TiO₂. When thecontent of MgO is above the said composition range, the withstandingcapacity against the switching surge will be less than 400 J/cc, andsuch a resistor is not suitable for the circuit breaker. When theadditives Sb₂ O₃, SiO₂, ZrO₂ and SnO₂ exceed said composition ranges,the resulting resistor has a resistivity higher than 4×10³ Ω.cm and alower withstanding capacity against the switching surge, and is notsuitable for the circuit breaker. A cause for these phenomena seems thtthe additives Sb₂ O₃, SiO₂, ZrO₂ and SnO₂ react mainly with the basiccomponent ZnO to form crystal grains such as Zn₇ Sb₂ O₁₂, Zn₂ SiO₄, Zn₂ZrO₄, and Zn₂ SnO₄ having electric resistances of 1×10.sup. Ω to 3×10¹³Ω, which are higher than that of the crystal grains ZnO and Zn₂ TiO₄formed from the basic composition of ZnO-TiO₂ -MgO, and the resultingresistors have an unbalanced distribution of crystal grains havingdifferent electric resistances.

Thus, a particularly preferable composition for the present resistorcontains 0.2 to 15% by weight (0.05 to 5% by mole) of Sb₂ O₃, 0.2 to 15%by weight (0.2 to 23% by mole) of SiO₂, 0.2 to 10% by weight (0.1 to 7%by mole) of ZrO₂ and 0.2 to 10% by weight (0.1 to 6% by mole) of SnO₂ onthe basis of the said basic components.

The present invention provides an oxide resistor, which is a compositeoxide sintered product comprising zinc oxide as the major component andother oxide than the zinc oxide as the minor component, characterized inthat the sintered product has a resistance-temperature coefficient ofwithin a range of ±5×10⁻⁴ Ω/°C. to -5×10⁻⁴ Ω/°C. at 20° to 500° C., aresistivity of 100 to 4,000 Ω-cm at 20° C., a withstanding capacityagainst the switching surge of 500 to 800 J/cc and a voltage non-linearcoefficient of 1.0 to 1.3 at 3×10⁻³ to 80 A/cm².

Furthermore, the present invention provides an oxide resistor, which isa sintered product comprising zinc oxide as the major component, 1 to20% by mole of magnesium oxide, and 0.1 to 20% by mole of at least oneof aluminum oxide, gallium oxide, lanthanum oxide and indium oxide,characterized in that a resistance layer having a lower resistivity thanthat of zinc oxide is formed between the crystal grains of zinc oxide.Particularly preferable are a sintered product comprising 70 to 92% bymole of zinc oxide, 3 to 10% by mole of magnesium oxide, and 5 to 15% bymole of aluminum oxide, and a sintered product comprising 68 to 90% bymole of zinc oxide, 3 to 10% by mole of magnesium oxide, 5 to 15% bymole of aluminum oxide, and 1 to 2% by mole of silicon oxide.

The present oxide resistor is a composite sintered product of crystalgrains of zinc oxide and crystal grains having an electric resistance of100 Ω to 4×10¹³ Ω, and having a grain boundary layer having a lowerelectric resistance than that of the crystal grains of zinc oxidebetween the crystal grains of zinc oxide. The sintered product may be ina plate form, a column form or a cylindrical form, and has electrodes onboth end surfaces. The electrodes in a metal film are formed onsubstantially entire surfaces by melt injection of a metal such as Al,while leaving some bare end portion on the end surfaces.

Between the individual crystal grains, there may be a grain boundarylayer having an electric resistance equal to that of the crystal grainsof zinc oxide. It is desirable that the crystal grains of zinc oxidecompound and other oxides than zinc oxide have an electric resistance of100 Ω to 4×10¹³ Ω, which is higher than that of zinc oxide. The zincoxide compound and other oxides than zinc oxide have the followingchemical formulae. That is, to much improve the linearity ofvoltage-current characteristics, at least one of ZnY₂ O₄, ZnGa₂ O₄,ZnLA₂ O₄, ZnAl₂ O₄, ZnIn₂ O₃, MgAl₂ O₄, MgY₂ O₄, MgGa₂ O₄, MgLa₂ O₄,MgIn₂ O₄, Al₂ O₃, Y₂ O₃, Ga₂ O₃, La₂ O₃ and In₂ O₃ is added to the baiscomponent MgO. To form these compounds, metal or semi-metal elememtssuch as aluminum (Al), yttrium (Y), gallium (Ga), lanthanum (La), indium(In), etc. are added to the main components ZnO and MgO. It is notpreferable to use Bi, because a layer of higher electric resistance isliable to be formed in the crystal grain boundary phase.

The raw materials for the present sintered product are zinc oxide (ZnO)and magnesium oxide (MgO) as the basic components, and the minorcomponent is selected from oxides of trivalent metals and semi-metalsother than ZnO and MgO, i.e. aluminum oxide (Al₂ O₃), yttrium oxide (Y₂O₃), gallium oxide (Ga₂ O₃), lanthanum oxide (La₂ O₃) and indium oxide(In₂ O₃). That is, the present inventors have investigatedcharacteristics of resistors comprising zinc oxide and magnesium oxideas basic components and further containing aluminum oxide, yttriumoxide, gallium oxide, lanthanum oxide, indium oxide, etc. to improve thelinearity of voltage-current characteristics of the resulting oxideresistors, and consequently have found that (1) the withstandingcapacity against the switching surge can be considerably increased to800 J/cc which is about 1.6 times that of the conventional resistor, (2)the resistance-temperature coefficient can be improved through a changefrom negative to positive by the content of magnesium oxide (MgO) in thebasic components zinc oxide (ZnO) and magnesium oxide (MgO), and (3) thelinearity of the resistivity and the voltage-current characteristics canbe improved by adding aluminum oxide (Al₂ O₃), yttrium oxide (Y₂ O₃),gallium oxide (Ga₂ O₃), lanthanum oxide (La₂ O₃), indium oxide (In₂ O₃),etc. to the basic components ZnO and MgO.

Preferable basic composition for the present resistor comprises 70 to99.7% by mole of zinc oxide, 0.1 to 10% by mole of magnesium oxide, and0.1 to 20% by mole of at least one of oxides such as Al₂ O₃, Y₂ O₃, Ga₂O₃, La₂ O₃ and In₂ O₃. The resistance-temperature coefficient can begreatly changed from negative to positive by the content of MgO, andwhen the content of MgO is above or below the said composition range,the resistance-temperature coefficient goes beyond the range of -1×10⁻³Ω/°C. to +4×10⁻³ Ω/°C. When the content of MgO exceeds the saidcomposition range, the withstanding capacity against the switching surgewill be less than 400 J/cc, and such a resistor is not suitable for thecircuit breaker. When the minor components of Al₂ O₃, Y₂ O₃, Ga₂ O₃, La₂O₃ and In₂ O₃ exceed the said composition range, the resistivity will behigher than 400 Ω.cm, and the withstanding capacity against theswitching surge will be lowered. Such a resistor is not suitable for thecircuit breaker. However, the resistivity can be controlled and thelinearity of the voltage-current characteristics can be improved byaddition of Al₂ O₃, Y₂ O₃, Ga₂ O₃, La₂ O₃, and In₂ O₃. A cause for thesephenomena seems that (1) the minor components of Al₂ O₃, Ga₂ O₃, In₂ O₃and La₂ O₃ react mainly with the basic component ZnO or MgO to formcrystal grains of ZnAl₂ O₄, ZnY₂ O₄, ZnGaO₄, ZnLa₂ O₄, ZnIn₂ O₄, MgAl₂O₄, MgY₂ O₄, MgGa₂ O₄, MgLa₂ O₄ and MgIn₂ O₄, whose electric resistancesrange from 50 .Ω to 4×10¹³ Ω, which are higher than those of crystalgrains of ZnO and MgO formed from the basic composition of ZnO-MgO, and(2) Al, Y, Ga, La and In are diffused into the crystal grains of ZnO toincrease the carrier concentration in the crystal grains of ZnO.

Particularly preferable composition for the present resistor comprises75 to 92.7% by mole of ZnO, 0.1 to 10% by mole of MgO, and at least oneof 0.2 to 20% by mole of Al₂ O₃, 0.2 to 10% by mole of Ga₂ O₃, 0.02 to5% by mole of In₂ O₃ and 0.1 to 10% by mole of La₂ O₃.

The present sintered resistor product is prepared, for example, bythoroughly mixing the said raw material oxide powders, adding water anda suitable binder such as polyvinyl alcohol to the mixture, pelletizingthe resulting mixture, molding the pellets in a mold, and sintering theresulting molding by firing in the atmosphere in an electric furnace ata temperature of 1,200° to 1,600° C. The sintered product is polished atboth end surfaces for forming electrodes, and the electrodes are formedon the polished end surfaces by plasma melt injection or baking. Toprevent any electric discharge along the side surfaces of the resistorduring the application, a ceramic layer or glass layer having a highresistivity may be provided on the side surfaces of the resistor. Thethus prepared resistor generally has a linearity, but when it shows anon-linearity, it is effective to break the high resistance parts(particularly the grain boundary layer) by application of a high voltagethereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 6 schematically show microstructures of oxide resistorsaccording to embodiments of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between thedensity of oxide resistor and the withstanding capacity against breakerswitching surge.

FIG. 3 is a diagram showing a relationship between the electric fieldintensity and the current density.

FIGS. 4 and 5 are cross-sectional view of an oxide resistor according toembodiments of the present invention.

FIG. 7 is an enlarged structural view of a resistor for the resistancemade in a gas circuit breaker (GCB) and FIG. 7A is a structural view ofthe gas circuit breaker.

FIG. 8 is a structural view of SF₆ gas-insulted neutral grounding (NGR).

PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLE 1

3,460 g of ZnO, 398 g of TiO₂ and 102 g of MgO as basic components and150 g of Sb₂ O₃, 60 g of SiO₂, and 62 g of ZrO₂ as additives wereexactly weighed out and wet mixed in a ball mill for 15 hours. Theresulting powdery mixture was dried, and 5% by weight of an aqueous 5wt. % polyvinyl alcohol solution was added thereto on the basis of thedried powdery mixture. The resulting mixture was pelletized. The pelletswere molded into a disc of 35 mm in diameter and 20 mm thick in a moldunder the molding pressure of 550 kg/cm². The molding was fired at1,400° C. in the atmosphere for 3 hours at an increasing and decreasingtemperature rate of 50° C./hr. Such crystal grains were formed in theresulting sintered product as ZnO crystal grains having an electricresistance of about 20 Ω, Zn.sub. 2 TiO₄ crystal grains having anelectric resistance of about 400 Ω, and Zn₇ Sb₂ O₁₂ crystal grains, Zn₂SiO₄ crystal grains and Zn₂ ZrO₄ crystal grains hasving electricresistances of 1×10⁷ to 3×10¹³ Ω.

Separately, crystallized glass powders of low melting point (ASF-1400glass of ZnO-SiO₂ -B₂ O₃ made by Asahi Glass K.K., Japan) were suspendedin an ethylcellulose butylcarbitol solution, and the resultingsuspension was applied to the side surface of the said sintered productto a thickness of 50 to 300 μm by a brush, and heated at 750° C. in theatmosphere for 30 minutes to bake the glass. The glass-coated sinteredproduct was polished at both end surfaces thereof each to about 0.5 mmby a lapping machine and washed with trichloroethylene. The washedsintered product was provided with Al electrodes to make a resistor. Thethus prepared resistor of the present invention was compared with theconventional resistor in the withstanding capacity against the switchingsurge, the resistance-temperature coefficient and the percent change inresistivity after heat treatment at 500° C. in the atmosphere. Theresults are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________             Characteristics                                                                     Withstanding capacity                                                                    Resistance-temperature                                                                    Percent change in resistivity                    Resistivity                                                                         against switching                                                                        coefficient (Ω/°C.)                                                          after heat treatment at                          (Ω · cm)                                                             surge (J/cc)                                                                             (20°-500° C.)                                                               500° C. in the atmosphere        __________________________________________________________________________                                          (%)                                     Present invention                                                                      500   810        +1 × 10.sup.-5                                                                       -2                                     Conventional                                                                           400   200        -9 × 10.sup.-2                                                                      +50                                     __________________________________________________________________________

It is seen therefrom that the present resistor has a very largewithstanding capacity against the switching surge, and smallerresistance-temperature coefficient and percent change in resistivityafter heat treatment at 500° C. than those of the conventional resistor,and thus is much distinguished.

In FIG. 1, the microstructure of the present resistor thus prepared isshown; in FIG. 2 a relationship between the density (g/cm²) of the thusprepared resistor and the withstanding capacity against the switchingsurge (J/cc) is shown; and in FIG. 3 the voltage-current characteristicsof the thus prepared resistor are shown.

Electric resistance of the formed crystal grains was measured by mirrorpolishing the sintered product, analyzing the polished surface by ascanning type electron microscope, forming microelectrodes on theindividual crystal grain surfaces, and measuring the current and voltageon the microelectrodes.

Embodiments of the present resistor structure are shown in FIGS. 4 and5, where schematic cross-sectional views of the present resistor areshown, and numeral 1 is a sintered product, 2 electrodes, and 3crystallized glass or ceramic film. As shown in FIG. 5, a hole 4 can beprovided at the center of the present resistor. In the case of SF₄gas-insulated neutral grounding, the electrodes are formed at innerpositions other than the peripheral side surface.

EXAMPLE 2

To investigate changes in the characteristics by a mixing ratio of basiccomponents, ZnO, TiO₂ and MgO, an amount x of TiO₂ and an amount y ofMgO in the mixing formula (100-x-y) ZnO-xTiO₂ -yMgO were changed eachbetween 0.1 and 40% by moles, and their mixing amounts were exactlyweighed out.

The weighed out raw material powders were mixed and fired at atemperature of 1,300° to 1,600° C. in the atmosphere for 4 hours in thesame manner as in Example 1, and the densities of the resulting sinteredproducts were 94 to 96% of the individual theoretical densities. Theresulting sintered porducts were polished at both end surfaces each toabout 0.5 mm by a lapping machine, ultrasonically washed intrichloroethylene. The washed sintered products were provided with Alelectrodes by Al melt injection to make resistors. The resistivity, thewithstanding capacity against the switching surge and theresistance-temperature coefficient of the thus prepared resistors areshown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________           Basic component                                                                    TiO.sub.2                                                                          MgO  Characteristics                                         Composition                                                                          ZnO  x    y    Resistivity                                                                         Withstanding capacity                                                                        Resistance temperature                                                        coeffi-                            No.    (mol. %)                                                                           (mol. %)                                                                           (mol. %)                                                                           (Ω · cm)                                                             against switching surge (J/cc)                                                               cient (Ω/° C.)                                                   (20°-500°            __________________________________________________________________________                                               C.)                                1      98.5 0.5  1    3.8 × 10                                                                      150            -8 × 10.sup.-2               2      98   1    1      4 × 10                                                                      300            --                                 3      94   5    1      5 × 10                                                                      520            -7 × 10.sup.-4               4      89   10   1      8 × 10                                                                      750            -8 × 10.sup.-4               5      79   20   1    8.2 × 10                                                                      410            -1 × 10.sup.-3               6      59   40   1     1.2 × 10.sup.2                                                               180            -6 × 10.sup.-1               7      89.8 10   0.2  6.9 × 10                                                                      730            -1 × 10.sup.-3               8      89.5 10   0.5  7.1 × 10                                                                      710            -3 × 10.sup. -4              9      89   10   1      8 × 10                                                                      750            -8 × 10.sup.-4               10     88   10   2    8.5 × 10                                                                      700            -4 × 10.sup.-5               11     85   10   5      9 × 10                                                                      650            -1 × 10.sup.-6               12     80   10   10    9.4 × 10.sup.2                                                               520            +1 × 10.sup.-5               13     75   10   15    1.2 × 10.sup.2                                                               420            +8 × 10.sup.-4               14     70   10   20   --    200            +5 × 10.sup.-3               15     60   10   30     9 × 10.sup.2                                                                110            +2 × 10.sup.-2               __________________________________________________________________________

It is seen from Table 2 that the resistors of composition Nos. 3 to 5and 7 and 13, that is, the compositions containing ZnO and 5 to 20% bymole of TiO₂ and the compositions containing 75 to 89.8% by mole of ZnOand 10% by mole of TiO₂, where 0.1 to 15% by mole of MgO is furthercontained, have distinguished characteristics such as a resistivity of40 to 120 Ω.cm, a withstanding capacity against the switching surge of400 to 750 J/cc, and a resistance-temperature coefficient within a rangeof -1×10⁻³ to +1×10⁻³ Ω/°C., and thus are suitable for the circuitbreaker.

Furthermore, it is seen from Table 2 that the withstanding capacityagainst the switching surge can be remarkably improved by adding TiO₂ toZnO as the basic components. However, if the content of TiO₂ is toolarge, e.g. 40% by mole (Composition No. 6), the withstanding capacityis 180 J/cc, which is smaller than 200 J/cc of the conventionalresistor. It is also seen therefrom that with increasing content of MgO,the resistance-temperature coefficient changes from negative to positiveand it can be made to fall within the range of ±1×10⁻³ Ω/°C. byselecting the optimum amount of MgO. Furthermore, it is seen therefromthat, even if the contents of TiO₂ and MgO are increased, theresistivity is kept in a range of about 4×10 to about 1.2×10² Ω.cm andundergoes no remarkable change. Thus, it is seen that a particularlypreferable compositon of basic components for a resistor for the circuitbreaker comprises 5 to 20% by mole of TiO₂ and 0.2 to 15% by mole ofMgO, the balance being ZnO.

EXAMPLE 3

ZnO was exactly weighed out from the range of 83 to 90% by mole, TiO₂from the range of 5 to 10% by mole, and MgO from the range of 5 to 7% bymole as basic components, while one of Sb₂ O₃, SiO₃, ZrO₂ and SnO₂ wasexactly weighed out each from the range of 0.2 to 30% by weight as anadditive thereto, and the basic components and the additive were mixedand kept at a temperature of 1,200° to 1,600° C. in the atmosphere for 4hours in the same manner as in Example 2 to make resistors. Theresistivity, the withstanding capacity against the switching surge, andthe resistance-temperature coefficient are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                                          Characteristics                                                                     Withstanding                                                                  capacity                                                                             Resistance                        Composition                          against                                                                              temperature                       Basic component                                                                              Additive              switching                                                                            coefficience               composition                                                                          ZnO  TiO.sub.2                                                                          MgO  Sb.sub.2 O.sub.3                                                                  SiO.sub.2                                                                         ZrO.sub.2                                                                         SnO.sub.2                                                                         Resistivity                                                                         surge  (Ω/°C.)       No.    (mol %)                                                                            (mol %)                                                                            (mol %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (Ω cm)                                                                        (J/cc) (20°-500°                                                        C.)                       __________________________________________________________________________    1      83   10   7    0.2             1 × 10.sup.2                                                                  730    +2 × 10.sup.-5       2      83   10   7    1               8 × 10.sup.2                                                                  770    +3 × 10.sup.-7       3      90   5    5    5               1.5 × 10.sup.3                                                                810    -2 × 10.sup.-5       4      90   5    5    10              2.4 × 10.sup.3                                                                680    -1 × 10.sup.-4       5      90   5    5    15              4 × 10.sup.3                                                                  420    -1 × 10.sup.-3       6      90   5    5    30              8 × 10.sup.3                                                                  170    -2 × 10.sup.-2       7      88   5    7        0.2         9 × 10                                                                        700    +1.5 × 10.sup.-5     8      88   5    7        1           4 × 10.sup.2                                                                  800    +2 × 10.sup.-6       9      88   5    7        5           1 × 10.sup.3                                                                  760    +1 × 10.sup.-7       10     85   10   5        10          2 × 10.sup.3                                                                  510    --                         11     85   10   5        15          3.5 × 10.sup.3                                                                300    -1 × 10.sup.-3       12     85   10   5        25          7 × 10.sup.3                                                                  190    -2 × 10.sup.-2       13     85   10   5            0.2     9.2 × 10                                                                      720    +1 × 10.sup.-5       14     85   10   5            1       6 × 10.sup.2                                                                  780    +8 ×  10.sup.-6      15     85   10   5            5       1.5 × 10.sup.3                                                                690    +4 × 10.sup.-6       16     85   10   5            10      3.4 × 10.sup.3                                                                640    +1 × 10.sup.-5       17     85   10   5            15      5.2 × 10.sup.3                                                                460    +1 × 10.sup.-3       18     85   10   5            20      2 × 10.sup.4                                                                  190    --                         19     88   5    7                0.2 1.2 × 10.sup.2                                                                700    +1.7 × 10.sup.-5     20     88   5    7                1   1 × 10.sup.3                                                                  760    +2 × 10.sup.-6       21     88   5    7                5   2.5 × 10.sup.3                                                                620    -1 × 10.sup.-4       22     85   10   5                10  4 × 10.sup.3                                                                  400    -9 × 10.sup.-4       23     85   10   5                15  6 × 10.sup.3                                                                  120    -2 × 10.sup.-3       24     85   10   5                30  1 × 10.sup.5                                                                   70    -3 × 10.sup.-2       __________________________________________________________________________

It is seen from Table 3 that the resistors containing 0.2 to 30% byweight of Sb₂ O₃, 0.2 to 25% by weight of SiO₂, 0.2 to 30% by weight ofZrO₂ or 0.2 to 30% by weight of SnO₂, that is, compositions Nos. 1 to 5,7 to 10, 13 to 16, and 19 to 22, have distinguished characteristics,i.e. a resistivity of 90 to 4×10³ Ω.cm, a withstanding capacity againstthe switching surge of 400 to 810 J/cc, and a resistance temperaturecoefficient within a range of -1×10⁻³ Ω/°C. to +1×10⁻³ Ω/°C., and aresuitable for the circuit breaker.

It is also seen from Table 3 that the resistivity is increased withincreasing contents of Sb₂ O₃, SiO₂, ZrO₂ and SnO₂ as the additive, butthe resistivity exceeds 4×10³ Ω.cm and becomes unsuitable for thecircuit breaker resistor, when the content of Sb₂ O₃ exceeds 30% byweight (Composition No. 6), the content of SiO₂ exceeds 25% by weight(Composition No. 12), the content of ZrO₂ exceeds 15% by weight(Composition Nos. 17 and 18), and the content of SnO₂ exceeds 15% byweight (Composition Nos. 23 and 24). When the contents of Sb₂ O₃, SiO₂,ZrO₂ and SnO₂ are too high as the additive, the withstanding capacityagainst the switching surge is lowered. For example, when the content ofSb₂ O₃ exceeds 30% by weight (Composition No. 6), the content of SiO₂exceeds 25% by weight (Composition No. 12), the content of ZrO₂ exceeds30% by weight (Composition No. 18), and the content of SnO₂ exceeds 15%by weight (Compositions Nos. 23 and 24), the withstanding capacity islowered to 70 to 190 J/cc, which is less than 200 J/cc of theconventional resistor.

The resistance-temperature coefficient tends to change from positive tonegative with increasing contents of Sb₂ O₃, SiO₂, ZrO₂ and SnO₂ as theadditive. For example, when the content of Sb₂ O₃ exceeds 30% by weight(Composition No. 6), the content of SiO₂ exceeds 25% by weight(Composition No. 12), the content of ZrO₂ exceeds 20% by weight(Composition No. 18), and the content of SnO₂ exceeds 15% by weight(Composition Nos. 23 and 24), the resistance-temperature coefficientwill be less than -1×10⁻³ Ω/°C., and thus such resistors are notsuitable for the circuit breaker.

It is seen therefrom that the preferable contents of Sb₂ O₃, SiO₂, ZrO₂and SnO₂ in the basic composition of ZnO-TiO₂ -MgO as a resistor for thecirsuit breaker are 0.2 to 15% by weight of Sb₂ O₃, 0.2 to 15% by weightof SiO₂, 0.2 to 10% by weight of ZrO₂, and 0.2 to 10% by weight of SnO₂.

EXAMPLE 4

3,420 g (84% by mole) of ZnO and 101 g (5% by mole) of MgO as the basiccomponents, and 510 g (10% by mole) of Al₂ O₃, 47 g (0.5% by mole) ofGa₂ O₃, and 369 g (0.5% by mole) of In₂ O₃ as the minor components wereexactly weighed out, and wet mixed in a ball mill for 15 hours. Then,the powdery mixture was dried, and 5% by weight of an aqueous 5 wt.%polyvinyl alcohol solution was added thereto on the basis of the driedpowdery mixture. Then, the mixture was pelletized, and the pellets weremolded into a disc, 35 mm in diameter and 20 mm thick in a mold underthe molding pressure of 450 kg/cm². The molding was sintered by firingat 1,350° C. in the atmosphere for 3 hours at the increasing anddecreasing temperature rate of 70° C./hr.

Crystal grains formed in the sintered product comprise crystal grains ofZnO having an electric resistance of about 10 to about 50 Ω, crystalgrains of ZnAl₂ O₃ having an electric resistance of about 70 to 100 Ω,and crystal grains each of ZnGa₂ O₄, ZnLa₂ O₄, ZnY₂ O₄, ZnIn₂ O₃, MgAl₂O₄, MgY₂ O₄, MgGa₂ O₄, MgLa₂ O₄, MgIn₂ O₃, Al₂ O₃, Ga₂ O₃, La₂ O₃ andIn₂ O₃ each having an electric resistance of about 700 to 4×10¹³ Ω.

The resulting sintered product was coated with crystallized glass of lowmelting point at the side surface in the same manner as in Example 1,and Al electrodes were likewise formed on both end surfaces thereof bymelt injection. The withstanding capacity for the switching surge, theresistance-temperature coefficient, the percent change in resistivityafter heat treatment at 500° C. in the atmosphere, and non-linearcoefficient α of voltage in the voltage-current characteristic betweenthe present resistor and the conventional resistor (carbon-dispersiontype ceramic resistor) are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________            Characteristics                                                                                                Non-linear coeffi-                                                                     Percent change in                         Withstanding capacity against                                                                Resistance-temperature                                                                    cient of voltage                                                                       resistivity after heat              Resistivity                                                                         switching surge                                                                              coefficient (Ω/°C.)                                                          3 × 10.sup.-3                                                                    treatment at                                                                  500° C. in the               (Ω · cm)                                                             (J/cc)         (20°-500° C.)                                                               ˜80 A/cm.sup.2                                                                   atmosphere                  __________________________________________________________________________                                                      (%)                         Present 550   800            +1.1 × 10.sup.-4                                                                    1.02      -2                         invention                                                                     Conventional*                                                                         400   500              -9 × 10.sup.-2                                                                    1.10     +50                         __________________________________________________________________________     *Carbon dispersion type ceramic resistor                                 

It is seen from Table 4 that the present resistor has a very largewithstanding capacity against the switching surge and a small non-linearcoefficient α of voltage, and thus is more distinguished than theconventional resistor.

The present resistor has a positive resistance-temperature coefficient,an AC withstanding capacity of at least 20 A at 100 μs and β of 0.9 to1.0 in the V-I characteristics.

The electric resistances of the individual crystal grains were measuredin the same manner as in Example 1.

The schematic microstructure of the thus prepared oxide resistor of thepresent invention is shown in FIG. 6. Provision of crystallized glassfilm or ceramic material film on the side surface of the sinteredproduct is made for preventing any electric discharge along the sidesurface during the application.

EXAMPLE 5

Basic component ZnO was exactly weighed out from the range of 65 to99.95% by mole, basic component MgO from the range of 0.05 to 20% bymole, and at least one of minor components Al₂ O₃, Y₂ O₃, La₂ O₃, In₂O₃, and Ga₂ O₃ from the range of 0.1 to 30% by weight. The weighed outraw material powders were sintered by firing at a temperature of 1,300°to 1,600° C. in the atmosphere for 3 hours in the same manner as inExample 1. The densities of the resulting sintered products were 95 to98% of the individual theoretical densities. The thus prepared sinteredproducts were polished on both end surfaces each to about 0.5 mm with alapping machine and ultrasonically washed in trichloroethylene. Thewashed sintered products were each provided with Al electrodes on bothend surfaces by Al melt injection to make resistors. The resistivity,the withstanding capacity against the switching surge, theresistance-temperature coefficient, and the non-linear coefficient α ofvoltage of the thus prepared resistors are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                                        Characteristics                                                                                   Non-                                                            Withstanding  linear                Composition                               capacity                                                                             Resistance-                                                                          coeffi-                     Basic                               against                                                                              temperature                                                                          cient of              Compo-                                                                              component Minor component           switching                                                                            coefficient                                                                          voltage               sition                                                                              ZnO  MgO  Al.sub.2 O.sub.3                                                                  Y.sub.2 O.sub.3                                                                   La.sub.2 O.sub.3                                                                  Ga.sub.2 O.sub.3                                                                  In.sub.2 O.sub.3                                                                  Resistivity                                                                         surge  (Ω/°C.)                                                                 10.sup.-3                                                                     A/cm.sup.2            No.   (mol %)                                                                            (mol %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (wt %)                                                                            (Ω · cm)                                                             (J/cc) 20-500° C.                                                                    -80                   __________________________________________________________________________                                                            A/cm.sup.2            1     99.95                                                                              0.05                     2 × 10                                                                        240    -4 × 10.sup.-3                                                                 1.8                   2     99.8 0.2                      6.5 × 10                                                                      395    -1 × 10.sup.-3                                                                 1.8                   3     99.5 0.5                      7 × 10                                                                        460    -3 × 10.sup.-4                                                                 1.9                   4     99   1                        8.2 × 10                                                                      620    +5 × 10.sup.-5                                                                 1.6                   5     95   5                        9 × 10                                                                        720    +6 × 10.sup.-4                                                                 1.9                   6     90   10                       1.2 × 10.sup.2                                                                490    +1.4 × 10.sup.-3                                                        .      2.0                   7     80   20                       5 × 10.sup.2                                                                  300    +4 × 10.sup.-3                                                                 1.6                   8     90   10   0.5                 9.1 × 10                                                                      570    +1.1 × 10.sup.-3                                                               1.8                   9     90   10   1                   2.4 × 10.sup.2                                                                700    +1 × 10.sup.-3                                                                 1.5                   10    90   10   5                   4 × 10.sup.2                                                                  780    +4.3 × 10.sup.-4                                                               1.1                   11    95   5    10                  8 × 10.sup.2                                                                  610    +8 × 10.sup.-5                                                                 1.02                  12    95   5    15                  1.5 × 10.sup.3                                                                520    +2 × 10.sup.-6                                                                 1.03                  13    95   5    20                  4 × 10.sup.3                                                                  380    +1 × 10.sup.-4                                                                 1.1                   14    95   5    30                  1 × 10.sup.5                                                                  150    -1 × 10.sup.-3                                                                 1.2                   15    93   7        0.2             9.5 × 10                                                                      690    +3 × 10.sup.-4                                                                 1.7                   16    93   7        0.5             1.5 × 10.sup.2                                                                540    +8 × 10.sup.-7                                                                 1.06                  17    93   7        1               5 × 10.sup.2                                                                  610    -2 × 10.sup.-5                                                                 1.03                  18    93   7        5               3.5 × 10.sup.3                                                                520    -5 × 10.sup.-5                                                                 1.15                  19    93   7        10              2 × 10.sup.6                                                                  300    -4 × 10.sup.-4                                                                 2.1                   20    90   10           0.1         1.4 × 10.sup.2                                                                540    +1.5 × 10.sup. -5                                                              1.4                   21    90   10           0.3         4 × 10.sup.2                                                                  620    +7 × 10.sup.-5                                                                 1.1                   22    90   10           0.5         6 × 10.sup.2                                                                  560    -2 × 10.sup.-6                                                                 1.02                  23    93   7            1           1 × 10.sup.3                                                                  500    +3 × 10.sup.-5                                                                 1.08                  24    93   7            5           3.5 × 10.sup.3                                                                430    -8 × 10.sup.-4                                                                 1.2                   25    93   7            10          8 × 10.sup.5                                                                  210    -4 × 10.sup.-3                                                                 1.8                   26    90   10               0.2     1.5 × 10.sup.2                                                                550    +2 × 10.sup.-4                                                                 1.7                   27    90   10               0.5     5 × 10.sup.2                                                                  600    +1.8 × 10.sup.-5                                                               1.08                  28    90   10               1       9 × 10.sup.2                                                                  540    -4 × 10.sup.-6                                                                 1.1                   29    85   15               5       1.8 × 10.sup.3                                                                500    +5 × 10.sup.-6                                                                 1.12                  30    85   15               10      4 × 10.sup.4                                                                  420    -3 × 10.sup.-4                                                                 1.2                   31    85   15               20      5 × 10.sup.7                                                                  260    -5 × 10.sup.-3                                                                 1.4                   32    93   7                    0.1 1.1 × 10.sup.2                                                                530    +1 × 10.sup.-5                                                                 1.3                   33    93   7                    0.3 6 × 10                                                                        600    +4 × 10.sup.-6                                                                 1.15                  34    93   7                    0.5 1 × 10.sup.2                                                                  580    +8 × 10.sup.-5                                                                 1.02                  35    85   15                   1   1.5 × 10.sup.2                                                                540    -3 × 10.sup.-6                                                                 1.09                  36    85   15                   5   5 × 10.sup.2                                                                  530    -5 × 10.sup.-4                                                                 1.1                   37    85   15                   10  3 × 10.sup.3                                                                  320    -1 × 10.sup.-5                                                                 1.16                  38    85   15                   20  1 × 10.sup.5                                                                  140    -3 × 10.sup.-3                                                                 1.3                   __________________________________________________________________________

It is seen from Table 5 that composition Nos. 10 to 12, 16 to 18, 21 to23, 27 to 29, and 32 to 26, that is, the resistors comprising 80 to92.9% by mole of ZnO and 5 to 1 5% by mole of MgO as the basiccomponents and one of 5 to 15% by weight of Al₂ O₃, 0.5 to 5% by weightof Y₂ O₃, 0.3 to 1% by weight of La₂ O₃, 0.5 to 5% by weight of Ga₂ O₃and 0.1 to 5% by weight of In₂ O₃ as the minor components have suchcharacteristics as a resistivity of 110 to 3,500 Ωcm, a withstandingcapacity against the switching surge of 500 to 780 J/cc, aresistance-temperature coefficient within a range of -5×10⁻ 4 Ω/°C. to+4.3×10⁻⁴ Ω/°C., and a non-linear coefficient α of voltage of 1.02 to1.3, and thus are distinguished as the resistors for the circuitbreaker.

Furthermore, it is seen from Table 5 that the withstanding capacityagainst the switching surge can be improved by adding MgO to ZnO.However, when the content of MgO is 20% by mole (Composition No. 7), thewithstanding capacity is 300 J/cc, which is smaller than 500 J/cc of theconventional resistor. By changing the content of MgO, theresistance-temperature coefficient changes from negative to positive,and can be made to fall, for example, within a range of -1×10⁻³ Ω/°C. to+4×10⁻³ Ω/°C.

Even if the content of MgO as the basic component is increased, theresistivity is kept to about 43 to about 500 Ω.cm, and undergoes nogreat change, but by addition of Al₂ O₃, Y₂ O₃, La₂ O₃, Ga₂ O₃, and In₂O₃ as the minor components thereto, the resistivity is considerablychanged in a range of 91 to 5×10⁻⁷ Ω.cm. Furthermore, the non-linearcoefficient of voltage can be considerably improved to 1.02 to 1.2 byselecting an optimum amount of the minor components Al₂ O₃, Y₂ O₃, La₂O₃, Ga₂ O₃, and In₂ O₃ to be added, but addition of too large an amountof the minor components Al₂ O₃, Y₂ O₃, La₂ O₃, Ga₂ O₃ and In₂ O₃ lowersthe withstanding capacity against the switching surge.

It is seen from the foregoing that a particularly preferable compositionfor a circuit breaker resistor comprises 95 to 85% by mole of ZnO and 5to 15% by mole of MgO as basic components and one of 5 to 15% by weightof Al₂ O₃, 0.5 to 5% by weight of Y₂ O₃, 0.3 to 1% by weight of La₂ O₃,0.5 to 5% by weight of Ga₂ O₃, and 0.1 to 5% by weight of In₂ O₃.

Example 6

In FIGS. 7A and 8, applications of the present oxide resistors preparedin Examples 1 and 4 each to a resistance in a gas circuit breaker (GCB)and an SF₄ gas-insulated neutral grounding (NGR), respectively, areshown. The resistor 5 of FIGS. 7A (shown in an enlarged view in FIG. 7)and 8 are in a cylindrical form shown in FIG. 5, where 6 is a bushing, 7a tank, 8 a condenser, 9 a breaker, 10 an oil dash-pot, 11 a piston forswitching operation, and 12 an air tank.

In FIG. 8, 17 is a bushing, 18 a tank and 19 a grounding terminal.

According to the present invention, a resistor can be made smaller insize and lighter in weight by using an oxide resistor having suchdistinguished characteristics as a very large withstanding capacityagainst the switching surge, a small non-linear coefficient of voltagein the voltage-current characteristics, a positive, smallerresistance-temperature coefficient, and a small percent change inresistivity after heat treatment at 500° C. in the atmosphere, asdescribed above.

What is claimed is:
 1. A composite sintered oxide resistor, obtained bysintering a powdery oxide mixture of zinc oxide as the main componentand other oxide of metal or semi-metal other than zinc oxide, free frombismuth oxide, so as to form crystal grains of zinc oxide and crystalgrains having an electric resistance of 200 Ω to 3×10¹³ Ω, free from agrain boundary layer having a higher electric resistance than that ofthe crystal grains of zinc oxide, the resistor being in a plate form andhaving electrodes at both end surfaces, and having a non-linearcoefficient of voltage of 1.0 to 1.3 at 3×10⁻³ to 80 A/cm².
 2. Acomposite sintered oxide resistor, obtained by sintering a powdery oxidemixture of zinc oxide as a major component and other oxide of metal orsemi-metal than zinc oxide, free from bismuth oxide powder, the resistorhaving a resistance-temperature coefficient of 5×10⁻⁴ Ω/°C. to -5×10⁻⁴Ω/°C. at 20° l to 500° C., a resistivity of 100 to 4,000 Ω at 20° C., awithstanding capacity against switching surge of 500 to 800 J/cc, and anon-linear coefficient of voltage of 1.0 to 1.3 at 3×10⁻³ to 80 A/cm².3. A composite sintered oxide resistor, which is obtained by sintering apowdery oxide mixture containing 0.1 to 10% by mole of magnesium oxide,0.1 to 20% by mole of at least one of yttrium oxide, aluminum oxide,gallium oxide, lanthanum oxide and indium oxide, and the balance beingzinc oxide, free of bismuth oxide, having zinc oxide grains and a grainboundary layer having a lower electric resistance than that of said zincoxide grains being formed between crystal grains, and the resistorhaving a non-linear coefficient of voltage of 1.0 to 1.3 at 3×10⁻³ to 80A/cm².
 4. A composite sintered oxide resistor according to claim 3,wherein the powdery oxide mixture comprises 70 to 92% by mole of zincoxide, 3 to 10% by mole of magnesium oxide, and 5 to 15% by mole ofaluminum oxide.
 5. A composite sintered oxde resistor, which is obtainedby sintering a powdery oxide mixture containing 68 to 90% by mole ofzinc oxide, 3 to 10% by mole of magnesium oxide, 5 to 15% by mole ofaluminum oxide and 1 to 2% by mole of silicon oxide, free from bismuthoxide, having crystal grains of zinc oxide and a grain boundary layerhaving a lower electric resistance than that of said crystal grains ofzinc oxide being formed between the crystal grains, and the resistorhaving a non-linear coefficient of voltage of 1.0 to 1.3 at 3×10⁻³ to 80A/cm².
 6. A gas circuit breaker with an oxide resistor, which comprisesan oxide resistor being a composite sintered oxide resistor according toany one of claims 1 to 5 and having a column or cylindrical form andelectrodes on both end surfaces excluding the side surface.
 7. A gascircuit breaker according to claim 6, wherein an insulating glass isprovided by baking on the entire side surface of the resistor.
 8. An SF₆gas-insulated neutral grounding with an oxide resistor, which comprisesan oxide resistor being a composite sintered oxide resistor according toany one of claims 1 to 5 and having a column or cylindrical form andelectrodes on both end surfaces excluding the side surface.
 9. An SF₆gas-insulated neutral grounding according to claim 8, wherein theelectrodes are formed at inner positions than the peripheral sidesurface.
 10. A composite sintered oxide resistor which comprisesindividual crystal grains including zinc oxide grains, obtained bysintering a powdery oxide mixture of zinc oxide as the main componentand other oxide of metal or semi-metal than zinc oxide, free frombismuth oxide, and a grain boundary layer having an electric resistanceequal to or lower than that of said zinc oxide grains being presentbetween said individual crystal grains, the resistor having a non-linearcoefficient of voltage of 1.0 to 1.3 at 3×10⁻³ to 80 A/cm².
 11. Acomposite sintered oxide resistor according to claim 10, wherein thegrain boundary layer between the individual crystal grains has anelectric resistance equal to that of the crystal grains of zinc oxide.12. A composite sintered oxide resistor according to claim 10, wherein avoid exists at the position corresponding to the grain boundary layerbetween the individual crystal grains.
 13. A composite sintered oxideresistor according to claim 10, wherein the metal or the semi-metalelement is titanium, silicon, antimony, zirconium, or tin.
 14. Acomposite sintered oxide resistor according to claim 10, wherein theindividual crystal grains further comprise grains having the followingchemical formula: Zn₂ TiO₄, Zn₂ SiO₄, Zn₇ Sb₂ O₁₂, Zn₂ ZrO₄ or Zn₂ SnO₄.15. A composite sintered oxide resistor according to claim 10, whereinthe resistor comprises crystal grains of zinc oxide and crystal grainsof a zinc oxide compound of other metal or semi-metal element than zinc.16. A composite sintered oxide resistor according to claim 15, whereinsaid powdery oxide mixture contains zinc oxide as the main component andat least one oxide selected from the group consisting of titanium oxide,silicon oxide, antimony oxide, zirconium oxide and tin oxide.
 17. Acomposite sintered oxide resistor according to claim 15, wherein saidpowdery oxide mixture contains zinc oxide as the main component,titanium oxide and magnesium oxide.
 18. A composite sintered oxideresistor according to claim 17, wherein said powdery oxide mixturefurther contains at least one element selected from the group consistingof antimony oxide, silicon oxide, zirconium oxide and tin oxide.
 19. Acomposite sintered oxide resistor according to claim 17, wherein saidpowdery oxide mixture contains zinc oxide as the main component,magnesium oxide and at least one oxide selected from the groupconsisting of aluminum oxide, gallium oxide, lanthanum oxide, indiumoxide and yttrium oxide.
 20. A composite sintered oxide resistoraccording to claim 19, wherein said powdery oxide mixture furthercontains silicon oxide.
 21. A composite sintered oxide resistor which isobtained by sintering a powdery oxide mixture consisting essentially of0.1 to 10% by mole of magnesium oxide and 0.1 to 20% by mole of at leastone yttrium oxide, gallium oxide, lanthanum oxide and indium oxide, thebalance being zinc oxide, having crystal grains of zinc oxide and agrain boundary layer having a lower electric resistance than that ofsaid crystal grains of zinc oxide being formed between crystal grains,and the resistor having a non-linear coefficient of voltage of 1.0 to1.3 at 3×10⁻³ to 80 A/cm².
 22. A composite sintered oxide resistor whichis obtained by sintering a powdery oxide mixture consisting essentiallyof 68 to 90% by mole of zinc oxide, 3 to 10% by mole of magnesium oxidepowder, 5 to 15% by mole of aluminum oxide and 1 to 2% by mole ofsilicon oxide, having crystal grains of zinc oxide and a grain boundarylayer having a lower electric resistance than that of said crystalgrains of zinc oxide being formed between crystal grains, and theresistor having a non-linear coefficient of voltage of 1.0 to 1.3 at3×10⁻³ to 80 A/cm².
 23. A composite sintered oxide resistor which isobtained by sintering a powdery oxide mixture consisting essentially of70 to 92% by mole of zinc oxide, 3 to 10% by mole of magnesium oxide and5 to 15% by mole of aluminum oxide, having crystal grains of zinc oxideand a grain boundary layer having a lower electric resistance than thatof said crystal grains of zinc oxide being formed between crystalgrains, and the resistor having a non-linear coefficient of voltage of1.0 to 1.3 at 3×10⁻³ to 80 A/cm².
 24. A composite sintered oxideresistor which is obtained by sintering a powdery oxide mixturecontaining zinc oxide, magnesium oxide, aluminum oxide, and siliconoxide, free of bismuth oxide, and having crystal grains of zinc oxideand a grain boundary layer having a lower electric resistance than thatof said crystal grains of zinc oxide being formed between crystalgrains.
 25. A composite sintered oxide resistor which is obtained bysintering a powdery oxide mixture consisting essentially of 65 to 94.8%by mole of zinc oxide, 0.2 to 154% by mole of magnesium oxide powder and5 to 20% by mole of titanium oxide, having crystal grains of zinc oxideand a grain boundary layer having an electric resistance equal to orlower than that of said crystal grains of zinc oxide being formedbetween crystal grains, and the resistor having a non-linear coefficientof voltage of 1.0 to 1.3 at 3×10⁻³ to 80 A/cm².
 26. A composite sinteredoxide resistor which is obtained by sintering a powdery oxide mixturecontaining 5 to 20% by mole of titanium oxide, 0.2 to 15% by mole ofmagnesium oxide, and 0.2 to 15% by weight of at least one oxide selectedfrom the group consisting of antimony oxide, silicon oxide, zirconiumoxide and tin oxide, the balance being zinc oxide, free from bismuthoxide, having crystal grains of zinc oxide and a grain boundary layerhaving an electric resistance equal to or lower than that of saidcrystal grains of zinc oxide being formed between crystal grains, andthe resistor having a non-linear coefficient of voltage of 1.0 to 1.3 at3×10⁻³ to 80 A/cm².
 27. A composite sintered oxide resistor according toclaim 26, wherein said powdery oxide mixture consists essentially of 65to 94.8% by mole of zinc oxide, 5 to 20% by mole of titanium oxide, 0.2to 15% by mole of magnesium oxide, 0.05 to 5% by mole of antimony oxide,0.2 to 23% by mole of silicon oxide, 0.1 to 7% by mole of zirconiumoxide, and 0.1 to 6% by mole of tin oxide.
 28. A gas circuit breakerwith an oxide resistor which comprises an oxide resistor being acomposite sintered oxide resistor according to claim 10 and having acolumn or cylindrical form and electrodes on both end surfaces excludingthe side surface.
 29. A gas circuit breaker according to claim 28,wherein an insulating glass is provided by baking on the entire sidesurface of the resistor.
 30. An SF₆ gas-insulated neutral grounding withan oxide resistor, which comprises an oxide resistor being a compositesintered oxide resistor according to claim 10 and having a column orcylindrical form and electrodes on both end surfaces excluding the sidesurface.
 31. An SF₆ gas-insulated neutral grounding according to claim30, wherein the electrodes are formed at inner positions other than theperipheral side surface.
 32. A composite sintered oxide resistoraccording to claim 10, wherein said powdery oxide mixture furthercontains magnesium oxide.